1. Field of the Invention
This invention relates to two fatty acid desaturase genes cloned from sorghum, the sorghum desaturase-2 gene, SbDES2 and the sorghum desaturase-3 gene, SbDES3, which consecutively convert 16:1 fatty acids to 16:3 fatty acids having a terminal double bond; a construct containing the genes and their promoter; a vector containing the genes; sorghum fatty acid desaturase-2 protein, SbDES2, and sorghum fatty acid desaturase-3 protein, SbDES3; a method of making SbDES2 protein and SbDES3 protein; a method of transforming plants; and transgenic plants which express SbDES2 and SbDES3 and thereby convert 16:1 fatty acids to 16:3 fatty acids having a terminal double bond in planta.
2. Description of the Relevant Art
Numerous plant species produce phytotoxic secondary metabolites, some of which may play a direct role in allelopathic interactions (Duke et al. 2002. J. Pesticide Science 27: 298-306; Inderjit and Duke. 2003. Planta 217: 529-539). These interactions often represent a form of chemical warfare occurring between neighboring plants competing for limited light, water, and nutrient resources (Inderjit and Duke, supra; Bais et al. 2004. Trends Plant Sci. 9: 26-32). Several sorghum species have been reported to produce phytotoxins which are exuded from root hairs into soil, which suppress the growth of surrounding weeds (Einhellig, F. A. 1996. Agronomy Journal 88: 886-893). Numerous studies have contributed to the discovery and identification of the chemicals that are responsible for this observed allelopathic inhibition. Studies on the biologically-active components of both water-soluble and water-insoluble exudates from root hairs of sorghum [Sorghum bicolor (L.) Moench] have shown the active role of hydrophobic exudates in the growth inhibition of lettuce seedlings (Lactuca sativa) as well as a number of important invasive weed species (Netzly and Butler. 1986. Crop Science 26: 775-778). The major component of biologically active root exudes from sorghum root hairs was identified as 2-hydroxy-5-methoxy-3-[8′Z,11′Z)-8′,11′,14′-pentadecatriene]-p-benzoquinone, referred to as sorgoleone (Chang et al. 1986. J. Amer. Chem. Soc. 108: 7858-7860). Considerable effort has subsequently been devoted to the study of the mechanism of phytotoxicity of sorgoleone (Einhellig et al. 1993. J. Chem. Ecology 19: 369-375; Einhellig et al. 1992. J. Chem. Ecology 18: 1-11; Gonzalez et al. 1997. J. Agric. Food Chem. 45: 1415-1421; Nimbal et al. 1996. J. Agric. Food Chem. 44: 1343-1347). Early reports on the phytotoxicity of sorgoleone indicated that it is a strong inhibitor of CO2-dependent oxygen evolution in plastids (Einhellig et al. 1993, supra). Further studies on its mode of action have demonstrated that this lipophilic benzoquinone affects both photosynthetic and mitochondrial electron transport (Gonzalez et al., supra; Rasmussen et al. 1992. J. Chem. Ecology 18: 197-207; Rimando et al. 1998. J. Natural Prod. 61: 927-930; Czarnota et al. 2001. Weed Technology 15: 813-825). Sorgoleone also inhibits plant p-hydroxyphenylpyruvate dioxygenase (HPPD; Meazza et al. 2002. Phytochem. 60: 281-288), an enzyme that catalyzes the conversion of p-hydroxyphenylpyruvate to homogetisate, which serves as an intermediate in the biosynthesis of plastoquinone (Que and Ho. 1996. Chem. Rev. 96: 2607-2624; Matsumoto et al. 2002. Weed. Biol. Manag. 2: 39-45). Inhibition of this enzyme leads to the disruption of the biosynthesis of carotenoids, resulting in foliar bleaching, characteristic of chlorophyll loss, and these symptoms are also observed in seedlings grown in the presence of living sorghum root systems (Nimbal et al. 1996. Pesticide Biochem. Physiol. 54: 73-83).
The herbicidal and allelopathic properties of sorgoleone make the isolation and characterization of the corresponding genes involved in sorgoleone biosythesis highly desirable, as manipulation of the pathway in sorghum, or genetic modification of other plant species using these genes could provide important insights into the underlying allelochemical interactions involved.